FORMATION OF CORAL REEFS AND ISLANDS. 199 



Besides a line of the greatest wave-action, we may also dis- 

 tinguish a height where this action is entirely null ; and it is 

 evident, from facts already stated, that the point will be found 

 somewhat above low-tide level. The lower waters of the surge, 

 instead of causing degradation, are accumulative in their ordin- 

 ary action, when the material exposed to them is movable : they 

 are constantly piling up, while the upper waters are eroding, 

 and preparing material to be carried off. The height at which 

 these two operations balance one another will be the height, 

 therefore, of the line of no degradation. As the sea at low 

 tide is mostly quiet, and the lower of the surging waters swell 

 on to receive the upper and parry the blow, and moreover, 

 there is next a return current outward, we should infer that the 

 line would be situated more or less above low tide, according 

 to the height of the tide and the surges accompanying it. We 

 are not left to conjecture on this point ; for the examples 

 presented by the shores of Australia and New Zealand afford 

 definite facts. Degradation has there taken place sufficient to 

 carry off cUffs of rock, of great extent; yet below a certain 

 level, the e has had Httle or no effect. This height, on the 

 eastern shores of Australia, is three feet above ordinary low 

 tide, and at New Zealand, above five feet. With regard to the 

 height varying with the tides, we observe that in the Paumotus, 

 where the water rises but two or three feet, the platform is 

 seldom over four to six inches above low tide, which is propor- 

 tionally less than at Australia and New Zealand, where the tide 

 is six and eight feet. From these observations it appears that 

 the height of no wave-action., as regards the degradation of a 

 coast under ordinary seas, is situated near one-fifth tide in the 

 Paumotus, and above half-tide at New Zealand, showing a 

 great difference between the effect of the comparatively quiet 

 surges of the middle Pacific, and the more violent of New- 

 Zealand. Within the Bay of Islands, where the sea has not its 

 full force, the platform, as around the " Old Hat," is but little 

 above low-water level. The exact relation of the height of the 

 platform to the height and force of the tides, and the force of 

 wave-action, remains to be determined more accurately by 



